1. Field of the Invention
This invention is directed to magnetic bubble domain devices, in general, and to a particular self-biased structure, in particular.
2. Prior Art
Magnetic bubble domain devices are becoming well known in the art. Conventional magnetic bubble domain devices require an external bias field to stabilize the bubble domains in the devices. The external bias field is usually supplied by a structure composed of permanent magnets and soft magnetic material. However, the external bias structure takes up a large amount of volume and weight in a magnetic bubble domain memory package. Thus, in various applications where package volume and weight must be minimized, the necessity of the external bias structure poses a hindrance.
Various approaches have been proposed to "internally" supply a bias field thereby eliminating the external bias structure. Two approaches are described in U.S. Pat. No. 3,529,303 and U.S. Pat. No. 3,714,640 to Bobeck. In essence, an effective bias field is supplied internally by a 180.degree. capping domain wall formed at the interface between a bubble domain film and a permanently magnetized layer which is exchange-coupled to the bubble film.
Epitaxially-grown self-biased structures were demonstrated by Uchishiba et al in a series of articles published in IEEE Transactions on Magnetics (Vol. MAG-9, pp. 381-385, 1973, Vol. MAG-10, pp. 480-483, 1974 and Vol. MAG-11, pp. 1079, 1081, 1975).
Although they have demonstrated with this structure a full device operation without an external bias field, a problem arises from the particular range of h/l, the normalized thickness of the bubble layer, required for full self-biasing. Namely, the above range of h/l falls below the preferred range used in most device applications.
A few suggestions have been made in the prior art for achieving a self-biased structure having h/l in the preferred range. U.S. Pat. No. 3,968,481 by Grundy and Lin suggests conditions under which h/l of the bubble layer can be brought into the preferred range. However, this teaching requires the assumption that the capping domain wall is formed in the bias layer if its l is greater than that of the bubble layer. The validity of this assumption is highly suspect. In fact, rigorous theoretical considerations and experimental evidence indicate that the capping domain wall formed as such is unstable.
Another approach has been suggested by Kobayashi et al (U.S. Pat. No. 4,059,828). In this approach, another (second) bias layer was added on top of the bubble layer. In this structure, the effective bias field is doubled.